1. Technical Field
The present invention relates generally to the modulation of signals and more particularly to a system for modulating a digital input signal onto a carrier to produce an analog output wherein the system operates at multiple data rates, employs a direct digital synthesis (DDS) operation to produce the analog output signal and provides filtering to reduce system noise and discontinuities in the modulated signal.
2. Related Art
It is well known to modulate signals of interest upon a carrier signal to produce a modulated signal. Typical applications of such modulation include wired and wireless communication systems. A wireless communication system employs a radio frequency (RF) signal upon which signals of interest are carried. Such signals of interest may include voice signals and digitized data. In such systems the data and/or voice signals are modulated onto the RF carrier at a transmitting location and are wirelessly transmitted to a receiving location. At the receiving location, the modulated RF signal is typically down mixed to an intermediate frequency and then demodulated to reproduce the signal of interest.
Many varied methods of modulation are commonly employed in such systems. Such modulation techniques include frequency modulation (FM), amplitude modulation (AM), quadrature amplitude modulation (QAM), phase shift keying (PSK) and frequency shift keying (FSK), along with various other modulation techniques. A particular modification of FSK modulation includes incorporating Gaussian filtering to produce a filtered modulated signal. Gaussian filtering operates to reduce side lobes and other non-carrier frequency components of the modulated signal. Such modulation technique is typically referred to as Gaussian frequency shift keying (GFSK).
In most communication systems, however, the modulation of signals is not a simple task. Problems with modulation result from various factors, not all of which may be contemplated in the design of the modulation circuitry. In one particular application wherein GFSK modulation is used, modulation causes the carrier signal to have a varying frequency, such variation dependent upon the bit pattern modulated onto the carrier. This frequency variation is controlled by the level of a baseband signal. This signal consists of symbols, each of which occupies the same amount of time and represents one or more bits. In a typical GFSK application, a positive peak may be three volts while a negative peak may be negative three volts. In two GFSK operation, wherein a single data bit is modulated onto each symbol period of the carrier signal, a logic zero is represented by a negative three volt peak while a logic one is represented by a positive three volt peak. In four GSFK modulation, wherein two data bits are modulated onto each symbol period of the carrier signal, additional levels are required to represent bit patterns (0,0), (0,1), (1,0) and (1,1). Likewise, in eight GSFK modulation, still further levels are required to represent the still greater number of bit patterns. Thus, the relative frequency of a particular symbol period of the carrier frequency which represents the bit pattern for the particular symbol must be accurately generated.
Thus there exists a need in the art for an improved modulation system supporting multiple data rates while providing smooth transitions between data rates in a modulated output signal.